Facilities in telecommunication networks can be divided into three different levels of call processing: service level, call control level and switching level. The switching level serves to provide the physical connections (i.e. “media”). The call control level is responsible for routing a call to a correct destination and it controls the switching level. The service level serves to produce “supplementary services” of the subscriber and possibly also part of the basic service and controls the call control level. In the existing telecommunication networks, a typical facility comprising a “supplementary service” is conditional call forwarding which requires analysis of the call situation and forward routing of the call in accordance with the service profile of the stored call forwarding. Conventionally, the facilities have not been divided into different levels but the facilities have been an inseparable part of the basic network, in which case changing or adding facilities has required, for example, updating of software in all exchanges of the network.
An intelligent network IN is a network architecture to be attached to a basic network (a fixed network or a mobile network, for example) enabling a facility to be divided into a service level and into an entity comprising a switching level and call control level. This is carried out by transferring service control from the telephone exchange to a separate functional unit of the intelligent network. Hereinafter, this unit will be called a service control point SCP. Consequently, the services are made independent of the operation of the basic network without a need to change the structure and software of the basic network when services are changed or added. Network nodes responsible for intelligent network interfaces are called service switching points SSP. Typically, an SSP is a network node responsible for connection setup, an exchange of the basic network, for example.
In the intelligent network, call control is based on a processing instance of a call and on a triggering mechanism contained in the processing instance. In the service switching point SSP, the call processing instance is usually modelled by at least one logical state model. The reason why a call must be modelled is that for the SCP to be able to control call setup in the SSP, the SCP must know the phase of the call. In the state model, the call processing procedures carried out in the SSP are grouped in rough phases. By means of modelling, both the SSP and the SCP implicitly know detection points DP, i.e. the phases of the call and possible interaction points in which the SCP can affect the call. The triggering mechanism is based on detecting the detection points DP and triggers, or transmitting requests at the detection point from the SSP to the SCP after the triggering condition has been fulfilled. The triggering condition comprises one or more conditions. There are two types of triggers: R or “Request” and N or “Notification”. When the SSP performs at the detection point DP an SCP request which is of the type R or “Request”, the SSP transmits the request to the SCP and remains waiting for a response from the SCP. In connection with the “Notification” type, the SSP only transmits the notification requested by the SCP, charging information, for example, at the detection point and immediately continues with the call setup.
A problem of the system described above is that the SSP must interrupt call processing after having sent the request and wait for instructions from the SCP even when the requested information necessary for controlling the SCP are available at an early phase (in a setup message, such as IAM, for example) but the instructions are only needed at a later phase of the call setup. An example of this is to route the call to C subscriber when the number of B subscriber is busy. If the transmission path between the SSP and the SCP is slow or several instructions are being requested sequentially, call setup or continuing with the call is slowed down since it is necessary to wait for messages to travel between the SSP and the SCP and for the SCP to be processed.